1. Field of the Invention
The present invention relates to a method and device for manufacturing UOE steel pipes, and more particularly to a method and device for manufacturing UOE steel pipes employing material, which exhibits a large springback after U-ing press, as a starting blank plate in association with a growing demand for increasing strength of steel pipes to be used.
2. Description of the Related Art
Conventionally, in manufacturing a large-diameter pipe, a method for manufacturing UOE steel pipes has been widely applied, wherein the manufacturing steps consist of “Crimping press”—“U-ing press”—“O-ing press”—“Internal/external welding”—“Expansion” in general.
FIG. 1 is a diagram explaining how a starting blank plate is deformed in “Crimping press”, “U-ing press” and O-ing press” among the overall manufacturing steps for UOE steel pipes. Firstly, in Crimping press, both width-wise edge parts of a starting blank plate (a heavy plate) are subjected to bending deformation to roughly match the outside diameter of the final steel pipe, in order to reduce a required working load in O-ing press as well as to prevent the generation of peaking phenomenon in O-ing press.
Next, in U-ing press, by employing a U-press having an open die, the flat plate formed by C-press is deformed into a U-like shape to yield a U-can 1 which can be conveyed to a unit of O-press. And then, in O-ing press, the U-can 1 thus conveyed is deformed into an O-like shape by employing an O-press having a closed die.
As shown in FIG. 1, subsequent to a series of “Crimping press”, “U-ing press” and O-ing press”, an internal & external welding is applied to make a primary steel pipe which does not satisfies the required dimensional accuracy in terms of the diameter and out-of-roundness. In this regard, the primary pipe is further expanded, so-called enlarging its bore in cold working condition, not only to correct the size of the primary steel pipe but also to reduce the internal tensile stress incurred by press forming as well as welding and to generate an internal compressive stress.
Meanwhile, for steel pipes for use in line pipe where UOE steel pipes are mostly adopted, the demand for high-strength line pipe capable of withstanding the high pressure-transportation is becoming high from the view point of enhancing the transportation efficiency of crude oil, natural gas or the like being obtained in oil and/or gas wells. For example, in association with increasing strength of steel pipes, it becomes possible to reduce the thickness of steel pipes to be laid, thus enabling costs for transportation and/or laying of steel pipes to be reduced.
At present, as steel pipes for use in line pipe, X42–X80 grades are specified in API (American Petroleum Institute) Standard. However, UOE steel pipes that are put into practice in line pipes remain to be made of X70 grade at most. Steel pipes of X80 grade are rarely used in actual line pipe, and only a few cases in actual application are reported thus far. This is attributed to the facts that, in making steel pipes having high-strength of X80 grade or more, it is very difficult to meet the required allowable yield-strength ratio, that is specified for X80 grade in API Standard, for example 93% or less, and press forming cannot be easily carried out.
In particular, as regards press formability, in association with increasing strength of material, the springback after U-ing press in UOE steel pipes becomes large, which leads up to cause a large opening width in the U-can. In the explanation hereinafter, an opening width of the U-can after U-ing press is simply referred to as “U-can width”.
FIGS. 2A and 2B are diagrams showing the relationship between the incidence of U-can width along with the U-can width itself and the die-diameter. Herein, the U-can width W is defined to represent a maximum opening width of the U-can 1. As shown in FIG. 2A, when the U-can width W is larger than the die-diameter of O-press 7, it is not possible to convey the U-can 1 to the proper position within the unit of the O-press 7 due to the interference with the dies 8. Therefore, as shown in FIG. 2B, U-ing press is repeated for plural times until the U-can width W should become smaller than the die-diameter of O-press 7, and then the U-can 1 has to be conveyed to the proper position within the unit of the O-press 7.
However, in a process applying a plurality of U-ing so that the U-can can be conveyed to the next step, it takes much longer time to perform U-ing press, thus resulting in the notable decrease in productivity. Further, since work hardening becomes eminent in the bottom part of the U-can as well as in the vicinity thereof subjected to plastic deformation in the plural U-ing process, the strength distribution in the circumferential direction after pipe making becomes uneven, and the yield-strength ratio of steel pipe also rises.
In order to reduce the U-can width after U-ing press, as shown in FIGS. 3A and 3B, in performing U-ing press by employing a punch 9 and a die 10 for U-press, a larger punch stroke of U-press than an ordinary punch stroke is occasionally applied for forming operation. FIG. 3A denotes the deformation in the case of the ordinary punch stroke, while FIG. 3B denotes the deformation in the case of the large punch stroke.
FIGS. 4A and 4B are diagrams showing the incidence of the U-can width after U-ing press in terms of the difference of the punch stroke. FIG. 4A denotes the incidence of the U-can width Wa in the case of the ordinary punch stroke in forming operation, while FIG. 4B denotes the incidence of the U-can width Wb in the case of the larger punch stroke in forming operation.
By applying a large punch stroke in a forming operation, it becomes possible for the U-can width Wb after U-ing press to be reduced. However, as the shape after forming becomes the configuration shown in FIG. 4B, it is likely that the out-of-roundness of steel pipes as end products becomes worse and the buckling of the U-can 1 during O-ing press may occur.
Meanwhile, as regards a means for conveying the U-can after U-ing press to the proper position within the unit of O-press, there is disclosed a method for conveying the U-can while preventing it from tilting by disposing a closing stand between the U-press and O-press units in Japanese Patent Publication No. 59-232620. However, the role of the side roll provided at said stand in the foregoing Japanese Patent Publication No. 59-232620 is merely to help conveying the U-can to the proper position within the unit of O-press. Thus, this cannot cope with the incidence of the U-can width in association with increasing strength of UOE steel pipes.